Wells are usually drilled through a formation that contains earth layers of different permeabilities. These wells are often lined with casing and cement that have perforations open along part of the well. Preferably, the fluid flow rate into and out of each layer should be about equal. If the permeability of one layer is anomalously high, the fluid flow rate in that layer is higher than in the other layers. Such a high permeability layer is called a "thief zone".
One of the best ways to improve sweep efficiency of a chemical flood or water flood is by sealing the thief zones deep inside the formation. Silva et al.'s mathematical model ("Waterflood Performance in the Presence of Stratification and Formation Plugging," SPE Paper No. 3556, 46th SPE Annual Meeting, New Orleans, Oct. 3 through 6, 1971) showed that sealing a thief zone at the wellbore was not enough. When the thief zone is sealed only near the wellbore, the flow behavior continued just beyond the sealed region as if no sealing had occurred. Unfortunately, most known methods tend to seal the thief zones only near the wellbore.
A more promising technique is to cause an in-situ change of the injection fluid in order to deeply penetrate the thief zone before sealing takes place. Two such commercial methods are now being used. One method is in-situ polymerization of carefully spaced slugs of monomer and catalyst. In theory, mixing these slugs deep inside the formation produces the desired polymer matrix and seals the thief zone. However, given the complexity of flow behaviors of slugs in a heterogeneous formation, it is difficult to place the polymer matrix where desired. The other method is time-delayed gelling of polymers. But the delay time may only be a few days (e.g., 48 to 72 hours for xanthan gum by chromium ions). Such short times prevent a deep penetration of the sealing fluid into the thief zone.
U.S. Pat. No. 3,620,302 to Robert Parsons teaches plugging thief zones with an inorganic silicate. An aqueous solution of the silicate is injected into the thief zones, in-situ combustion is generated in a nearby zone, and that combustion is sustained until enough heat is transferred to the thief zones to cause the silicate to intumesce and seal the thief zones. Unfortunately, such in-situ combustion cannot be used in most fields.
U.S. Pat. No. 3,669,188 to Roy Coles et al. teaches using a plugging fluid that reacts to deposit a plugging material as temperature increases. The thief zone near the well is heated so that it is hotter than the surrounding regions, then the plugging fluid is injected into that zone, and then unreacted plugging fluid is displaced after plugging has occurred. Preferably, the plugging fluid is an aqueous solution of a metal and a reactant. The metal precipitates as a gelatinous metal hydroxide and the reactant increases the solution pH to cause that precipitation. Preferably, slugs of hot water are used to heat the thief zone. This injected hot water, however, may lose a substantial amount of energy, making the process less effective deep inside the thief zone, where plugging is needed the most. Basically, this method was designed to apply in the "near well" region (less than twenty feet from the wellbore).
Electrical heating has been patented in U.S. Pat. Nos. 3,857,776 and 4,013,538. Such methods have been discounted as drive methods for tar sands recovery in that the temperature rise in the formation during that type of heating appears to be confined to a narrow path from one electrode to the other.